Cardiac muscle tissue

Textus muscularis striatus cardiacus

Also known as: cardiac muscle, cardiomyocytes, heart muscle

Striated, involuntary muscle of the heart wall. It is made of branched cells with one or two central nuclei, joined end-to-end by intercalated discs.

Overview

Cell size≈10–25 µm × 50–100 µm

Nuclei per cell1–2, central

Mitochondria≈30% of cell volume

Cardiac muscle cells (cardiomyocytes) are short (≈50–100 µm), branched, striated cells with 1–2 central nuclei. They are joined end-to-end by intercalated discs, which contain mechanical junctions (fascia adherens and desmosomes) and gap junctions. The gap junctions couple the cells electrically, so the myocardium behaves as a functional syncytium. Sarcomeres are organised as in skeletal muscle. However, T-tubules are wider, lie at the Z discs, and usually form dyads with a single SR cisterna rather than triads.

Contraction is involuntary and starts spontaneously: pacemaker cells of the conduction system depolarise on their own. The autonomic nervous system and hormones adjust rate and force, but are not needed for contraction to occur. Excitation–contraction coupling relies on Ca²⁺-induced Ca²⁺ release. Ca²⁺ entering through L-type channels during the plateau of the action potential triggers RyR2 to release Ca²⁺ from the SR. The long plateau and refractory period prevent summation and tetany, which is essential for the heart to work as a pump. Cardiomyocytes are packed with mitochondria (≈30% of cell volume) and depend almost entirely on aerobic metabolism. Their capacity to regenerate is very limited.

Location

Myocardium of the atria and ventricles, and the myocardial sleeves extending into the roots of the great veins (pulmonary veins, venae cavae).

Parts & subdivisions

Intercalated discs

Step-like junctions between cells. Their transverse parts contain fascia adherens and desmosomes (force transmission); their lateral parts contain gap junctions (connexin 43; spread of the electrical impulse).

Dyads

A T-tubule paired with one terminal SR cisterna, located at the Z line.

Pacemaker and conducting cells

Modified cardiomyocytes of the SA node, AV node, bundle of His and Purkinje fibres.

Anatomy

Histology

Branched striated fibres with central nuclei, often surrounded by a pale perinuclear zone that contains lipofuscin in older people. Intercalated discs appear as dark transverse lines, and capillaries are abundant in the endomysium. Purkinje fibres are larger and paler, packed with glycogen.

Development

Develops from splanchnic mesoderm of the cardiogenic field (first and second heart fields). Cardiomyocytes divide until shortly after birth; afterwards the heart grows mainly by cell hypertrophy.

Function

Rhythmic, coordinated contraction that pumps blood

Generation of its own rhythm (automaticity) and conduction of impulses

Secretion of natriuretic peptides (atrial ANP, ventricular BNP)

Physiology

The ventricular action potential has phases 0–4, with a plateau (phase 2) maintained by L-type Ca²⁺ current. Contractile force rises with preload (Frank–Starling mechanism) and with sympathetic β₁ stimulation (more Ca²⁺ entry and faster SERCA reuptake through phospholamban phosphorylation).

Clinical significance

Myocardial infarction

Ischaemic cardiomyocytes die and are replaced by fibrous scar because regeneration is minimal. Cardiac troponin I or T released from necrotic cells is the standard biomarker.

Cardiomyopathies

Mutations in sarcomere genes such as MYH7 (β-myosin heavy chain) and MYBPC3 cause hypertrophic cardiomyopathy. Truncating mutations of titin (TTN) are the commonest genetic cause of dilated cardiomyopathy.